Absorber element for solar high-temperature heat generation, and a method for its production

ABSTRACT

The invention relates to an absorber element for solar high-temperature heat generation, having a light focusing element, an outer tube composed of a translucent material and an absorber which is arranged in it. The absorber is surrounded by at least one reflector channel having an opening gap. The focal line of the light focusing unit runs on the centre axis of the outer tube and the absorber does not lie on the centre axis of the outer tube. The opening gap of the reflector channel, through which the solar rays fall on the absorber, lies on the centre axis of the outer tube, and hence on the focal line.

DESCRIPTION

[0001] The invention relates to an absorber element for solarhigh-temperature heat generation, of the type mentioned in theprecharacterizing clause of claim 1. Absorber elements such as these aregenerally known. The invention also relates to a method for producingsuch an absorber element.

[0002] In solar-thermal power stations, the radiation energy from thesun is concentrated by means of mirror systems, preferably withreadjustment, and is used for heating a heat carrier medium. In onegenerally known design, the heat carrier medium flows through longabsorber tubes, which are arranged at the focus of parabolic groovecollectors.

[0003] Instead of using reflecting systems, the radiation energy canalso be concentrated by using refracting systems (convergent lenses).One example of a linearly optically concentrating system such as this isdisclosed in U.S. Pat. No. Specification 4,287,881, dated Sep. 8, 1981.

[0004] Solar collectors, in which the actual absorber is surrounded byelements which reflect the radiation back to it, are disclosed, forexample, in French Patent Specification 555 420, U.S. Pat. Nos.Specifications 4,300,538, 4,440,155, 4,512,335 and German Publications27 57 155 and 30 20 310.

[0005] One problem with solar high-temperature heat generation is theheat losses that result from radiation emitted by the absorber tube.Owing to the high temperature, these heat losses are considerable. Inorder to reduce the heat losses, it is generally known for the actualabsorber tube to be arranged in an outer tube. Although this reduces theheat losses, they are, however, still sufficiently high that theefficiency of solar high-temperature heat generation remains well belowthe theoretically achievable level.

[0006] The object of the invention is thus to provide an absorberelement for solar high-temperature heat generation, in which the heatlosses are minimal. A further aim is to provide a simple method forproducing an absorber element such as this.

[0007] According to the invention, this object is achieved by theabsorber element described in claim 1. Advantageous refinements of theabsorber element according to the invention are specified in thedependent claims. Claim 13 describes one preferred method for producingthe absorber element.

[0008] The reflector channel which, according to the invention, isarranged around the absorber reflects the long-wave thermal radiation,which is radiated from the absorber, back to it, and thus minimizes theradiation heat losses.

[0009] At least on the inner face which points towards the absorber, thereflector channel is preferably formed from a small number ofessentially planar surfaces. The majority of the thermal radiation whichthe absorber emits over an angle range of 180° is largely reflecteddirectly back to the absorber from the straight reflector channel walls,and is not lost by multiple reflection on curved reflector channelwalls.

[0010] The opening gap or slit in the reflector channel runs on thefocal line of the light focusing unit. The focal line lies on the centreaxis of the outer tube. Since the solar rays are focused by the lightfocusing unit onto the centre axis of the outer tube, they strike theouter tube at right angles and are not refracted by it. The absorberwith its absorber tube and curved absorber plates can thus be stationaryand firmly mounted. The solar rays, which diverge after the centre axis,completely strike an absorber channel, which is formed by the absorbertube and the absorber plates. No sunlight therefore strikes the wall ofthe reflector channel, which is optimized to reflect heat rays from theabsorber.

[0011] The solar rays thus effectively heat the heat carrier medium inthe absorber tube, for example in order to produce steam. The compactshape of the absorber element according to the invention allowshigh-temperature steam to be produced with low losses in small units.Even in regions in which the sunlight is relatively weak, the use ofsmall units for high-temperature heat generation is thus technically andfinancially of interest. The steam can be used with high efficiency in acirculating process for electricity generation. Solar generation canthus be used at a high temperature for heating purposes, even in winter.

[0012] The production method according to the invention allows a minimumopening gap to be produced in the reflector channel, while compensatingfor the manufacturing and assembly tolerances of the light focusingunit. This results in a gap geometry which is optimally matched to therespective structure. The manufacturing complexity for the lightfocusing unit and for the absorber unit is reduced, and the thermallosses during operation are minimized.

[0013] Embodiments of the invention will be explained in more detail inthe following text using the drawing by way of example, in which:

[0014]FIG. 1 shows a cross section through an absorber element;

[0015]FIG. 2 shows the absorber element in an arrangement with groovereflectors;

[0016]FIG. 3 shows the absorber element in an arrangement withconvergent lenses.

[0017]FIG. 1 shows a cross section through an absorber element for solarhigh-temperature heat generation. The solar rays 10 coming from the sunare focused by a linear light focusing unit 60 (which is shown in FIG.2) onto the centre line of an outer tube 20. The centre line of theouter tube 20 thus coincides with the focal line of the light focusingunit. The outer tube 20 is preferably composed of glass. In general, theouter tube 20 is composed of a translucent material, through whichshort-wave radiation passes well but long-wave thermal radiation doesnot pass well. These requirements are satisfied by glass.

[0018] There is a vacuum in the interior of the outer tube 20. The rays,which diverge again behind the focal line, strike an absorber 30 there,which is arranged eccentrically with respect to the centre axis of theouter tube 20. The absorber 30 has a high absorption capability on theirradiated face, and has a low emission capability on the opposite face,in order to absorb a large amount of solar radiation and to emit littlethermal radiation.

[0019] The absorber 30 is fixed in the outer tube 20. The absorber 30comprises an absorber tube 32, in which a heat carrier mediumcirculates, and absorber sheets or absorber plates 34 which are fittedto the absorber tube 32. The absorber sheets or absorber plates 34 arepreferably welded to the absorber tube 32, in order to ensure good heattransmission from the absorber plates 34 to the absorber tube 32. Theabsorber plates 34 are curved such that the incident solar rays 10 arevirtually completely received and absorbed. The high degree ofabsorption is achieved by multiple reflection and absorption processesfor obliquely incident solar rays 10 along the curved surface on theinner face of the absorber plates 34.

[0020] The absorber 30 is surrounded by a reflector channel 40, whichreflects the thermal radiation coming from the absorber 30 back to it.The surface of the reflector channel 40 has a low emission andabsorption capability. The reflector channel 40 has an opening gap 42,through which the solar rays 10 enter the channel 40, on the focal line.Since the focal line runs in the opening gap 42, the opening gap 42 maybe narrow. The heat losses through the opening gap 42 are thus small.Apart from the opening gap 42, the reflector channel 40 is closed allthe way round.

[0021] On the inner face pointing towards the absorber 30, the reflectorchannel 40 is formed from a small number of essentially planar surfaces,in order to ensure that the thermal radiation originating from theabsorber 30 is reflected back as directly as possible. In cross section,as can be seen in FIG. 1, the reflector channel 40 thus represents arectangular or trapezoidal structure, in which the face of the reflectorchannel 40 in which the focal line runs can be bent slightly inwardstowards the absorber 30 in order to achieve an optimum radiation profileon precisely this focal line, that is to say on the opening gap 42.

[0022] On its inner face with this bend, the reflector channel 40 thushas five essentially planar surfaces, and without a bend it has foursuch surfaces.

[0023] The reflector channel 40 is preferably coaxially surrounded by afurther, outer reflector channel 50, which has the same surfacecharacteristics as the inner reflector channel 40 and is connected in aninterlocking manner to it at a small number of points by means of poorlythermally conductive elements, composed, for example, of ceramic. Theouter reflector channel 50 on the face which the light strikes has anopening gap 52 which runs parallel to the opening gap 42 in the innerreflector channel 40 and is somewhat broader than it, in order to avoidblocking the solar rays 10.

[0024] The statements which have been made with regard to the innerreflector channel 40 apply equally to the interior of the outerreflector channel 50, that is to say the inner face of the outerreflector channel 50, which points towards the absorber 30, as well, isformed by a small number of essentially planar surfaces. However, theouter reflector channel 50 is circular on the outside, and can berotated together with the inner reflector channel 40 about the centreaxis of the outer tube 20. The reflector channels 40, 50 are positionedin rotation via magnets which are arranged outside the outer tube 20, inorder to match their position to the incidence of the light, dependingon the time of day. The magnets are mounted on a holding structure,which is arranged such that it can rotate about the outer tube axis. Thelight focusing unit is also mounted on the holding structure. Theholding structure is positioned in rotation by an electric motor ormotors. An optoelectronic sensor which is rotated together with theholding structure controls the rotational positioning for readjustmentto match the light incidence direction.

[0025] The reflector channels 40, 50 have the function of reflectinglong-wave thermal radiation which is emitted from the absorber 30 backto it, and thus of minimizing the heat losses caused by radiation.Electrochemically plated metal surfaces are able to reflect the majorityof long-wave radiation. The absorber tube 32 together with the absorberplates 34 which are mounted on it is arranged firmly in the outer tube20 and is not involved in the positioning movements of the reflectorchannels 40, 50. The absorber plates 34 absorb the short-wave solarradiation and prevent the solar radiation from striking the innerreflector channel 40 directly, and being absorbed there.

[0026]FIG. 2 shows the arrangement of the outer tube 20 with theabsorber 30 on a light focusing unit 60 with sheet deflection mirrors 62and a parabolic groove mirror 64. The mirrors 62, 64 are mounted on aholding structure such that they can rotate about the axis of the outertube 20. The sheet deflection mirrors 62 are arranged in thelongitudinal direction of the outer tube 20, and are rotatably mountedat an angle of 90° with respect to the outer tube 20. They deflectobliquely incident rays such that they are always incident at rightangles to the incidence plane of the parabolic groove mirror 64. Inconsequence, the rays always enter the outer tube 20 through its wall atright angles, without any major reflection losses. The sheet deflectionmirrors 62 are positioned in rotation by an electric motor or motors,and their positions are controlled as a function of the time of day.

[0027]FIG. 3 shows an alternative arrangement of the outer tube 20 withthe absorber 30 on a light focusing unit with a linear convergent lens66. The convergent lens 66 is mounted on a holding structure such thatit can rotate with respect to the outer tube axis. The focal line of theconvergent lens 66 lies on the outer tube axis.

[0028] In the described absorber element, the opening gap 42, 52 of thereflector channel 40 or of the reflector channels 40, 50 lies on thefocal line of the light focusing unit 60, and on the centre axis of theouter tube 20. Since the solar rays are always focused on the centreaxis of the outer tube 20 by the light focusing unit 60 which isreadjusted to match the sun, they pass through the outer tube 20 atright angles, and are not refracted there. The absorber tube 32 and theabsorber plates 34 which are thermally connected to it form an absorberchannel, which is mounted firmly. The solar rays, which diverge onceagain after the centre axis, all enter this absorber channel. Thereflector channel 40 or the reflector channels 40, 50 is or arereadjusted by external magnetic forces, for example, with the outer tube20 being stationary.

[0029] The described arrangement can be used for solar high-pressuredirect vaporization for electricity generation in small power stations.

[0030] Water is used as the heat carrier medium for this purpose. Thewater is heated in an absorber tube 32 of an absorber element which isused as a feed water heater, and is vaporized and superheated in two ormore downstream elements, in order to produce superheated high-pressuresteam. The elements are arranged geographically offset with respect toone another, so that the superheated steam enters the upper elements.Two or more elements are in each case combined to form a module. Theelements which can rotate are accommodated in the holding structure forthe module. The absorber tubes of all the elements are expediently ofthe same length. However, since the vaporization process requires morethermal power than that required to heat the feed water, two absorberelements connected in parallel are used as evaporators, and some of theheated feed water is supplied to each of them via a distributor. In asimple case, the distributor may have a throttle valve in each feed lineof the two absorber elements which form the evaporator, in order tosupply the same amount of feed water to both. If these two absorberelements are fitted at different geodetic heights, the distributor mayalso have an overflow container for the heated feed water, whichcontains two overflows which are separate but are located at the sameheight and above the two absorber tubes of the evaporator, and fromwhich the feed water flows away in a corresponding manner to the twoabsorber tubes. Both absorber tubes thus receive the same amount of feedwater, despite being at different geodetic heights. The steam from twoor more modules is carried in a closed circuit, and is expanded in aprocess machine in order to generate electricity. The expanded steam isliquefied, with heat being emitted to the environment, and is fed backto the modules via a feed water pump.

[0031] In low-power systems, a reciprocating piston motor with steppedpistons can be used as the process machine. A multistage reciprocatingpiston motor can adapt itself well to changing load requirements. Forhigher power levels, steam turbines may be used, although they do notreact as quickly to load changes owing to the changing energy supply.

[0032] The described absorber element can advantageously be produced byassembling the absorber element completely, but with at least the wallof the inner reflector channel 40 still not having an opening gap 42.Parallel laser light is then injected via the light focusing unit 60and, focused in this way, burns out the opening gap 42 from the wall ofthe reflector channel 40. This method results in an opening gap whichtakes account of the manufacturing and assembly tolerances of the lightfocusing unit with reduced manufacturing effort. The minimal size of theopening gap minimizes the thermal losses.

[0033] The opening gap 52 in the wall of the outer reflector channel 50can also be produced in the same way.

What is claimed is:
 1. An absorber element for solar high-temperatureheat generation, having a light focusing element, an outer tube composedof a translucent material and an absorber which is arranged in the outertube and to which the solar rays are passed by the light focusingelement, wherein the absorber is surrounded by at least one reflectorchannel, whose surface has a low emission and absorption capability, andwhich reflects the heat radiation which originates from the absorberback to the absorber; the focal line of the light focusing element lieson the centre axis of the outer tube, and the absorber does not lie onthe centre axis of the outer tube; an opening gap in the reflectorchannel runs on the centre axis of the outer tube and the solar raysfall on the absorber through this opening gap; and the absorbercomprises an absorber tube, through which a heat carrier mediumcirculates, and absorber plates which are mounted on the absorber tube,with the absorber plates being curved such that they essentiallycompletely absorb the solar rays which are incident on them through theopening gap.
 2. An absorber element according to claim 1, wherein theinner face of the reflector channel, which points towards the absorber,is formed from a small number of essentially planar surfaces.
 3. Anabsorber element according to claim 1, wherein an outer reflectorchannel, which surrounds an inner reflector channel, is providedcoaxially with respect to the inner reflector channel and hasessentially the same characteristics as the inner reflector channel. 4.An absorber element according to claim 3, wherein the inner and theouter reflector channels are jointly readjusted to track the sunlight.5. An absorber element according to claim 4, wherein the inner and theouter reflector channels are readjusted by means of magnets which aremounted on a holding structure outside the outer tube.
 6. An absorberelement according to claim 1, wherein the absorber together with theabsorber tube and the absorber plates is firmly mounted, and is notreadjusted.
 7. An absorber element according to claim 1, wherein thelight focusing unit comprises sheet deflection mirrors and parabolicgroove mirrors.
 8. An absorber element according to claim 1, wherein thelight focusing unit has at least one linear convergent lens.
 9. Anabsorber element according to claim 1, wherein the outer tube iscomposed of glass.
 10. An absorber element according to claim 1, whereinthe heat carrier medium is water.
 11. An absorber element according toclaim 10, wherein the steam which is generated in such absorber elementsis supplied to a process machine for electricity generation.
 12. Anabsorber element according to claim 11, wherein the process machine is areciprocating piston motor with stepped pistons.
 13. A method forproducing an absorber element according to claim 1, wherein the absorberelement is first of all assembled, but with the wall of the reflectorchannel initially not having an opening gap yet, and parallel laserlight is then injected via the light focusing unit and burns the openinggap out of the wall of the reflector channel.